Unlicensed spectrum may be used as a complement to the licensed spectrum and/or may allow completely standalone operation.
Ongoing work in 3GPP Third Generation Partnership Program (3GPP) relates to “Licensed-Assisted Access” (LAA) and intends to allow UMTS-LTE (Universal Mobile Telecommunication System, Long Term Evolution; hereinafter “LTE”) compliant equipment to also operate in frequency ranges (bands) denoted as unlicensed radio spectrum. LTE in unlicensed spectrum may be denoted LTE-U. When the unlicensed spectrum is used as a complement to the licensed spectrum, devices typically connect in the licensed spectrum (to a primary cell or PCell) and use carrier aggregation to benefit from additional transmission capacity in the unlicensed spectrum (to a secondary cell or SCell). A typical carrier aggregation (CA) framework allows aggregation of two or more carriers with the condition that at least one of the carriers (or frequency channels) is in the licensed spectrum and at least one of the carriers is in the unlicensed spectrum. Further evolution of the LAA feature (which only supports downlink (DL) traffic) has been specified in terms of enhanced License Assisted Access (eLAA), which includes the possibility to also schedule uplink (UL) traffic on the secondary carriers (in the unlicensed bands). Candidate bands for LTE operation in the unlicensed spectrum include the 5 GHz band, the 3.5 GHz band, etc.
In a typical standalone (or completely unlicensed spectrum) mode of operation, one or more carriers, all of which are in the unlicensed spectrum, are used. In parallel to the work within 3GPP, the MulteFire Alliance (MFA) also has ongoing work to standardize a system that allows the use of standalone primary carriers within unlicensed spectrum. The resulting MulteFire 1.0 standard supports both UL and DL traffic.
Traditionally, the unlicensed 5 GHz spectrum is mainly used by equipment implementing the IEEE 802.11 Wireless Local Area Network (WLAN) standard. This standard is known under its marketing brand “Wi-Fi” and allows completely standalone operation in the unlicensed spectrum.
Internet of Things (IoT) can be considered a fast evolving market within the telecommunications realm, and 3GPP standards currently offer some different variants supporting IoT services (e.g. eMTC: enhanced machine type communication, NB-IoT: narrow band IoT, and EC-GSM: extended coverage GSM (Global System for Mobile communication)). Some use cases, e.g. where outdoor macro eNodeBs communicate with IoT devices deep inside buildings, may require standardized coverage enhancement mechanisms (an example of increased maximum communication distance range). Some possible features/requirements of IoT devices include power saving mode, long battery lifetime, reduced modem complexity, coverage enhancement, and reduced user equipment (UE) bandwidth (e.g. 1.4 MHz in downlink) In order to achieve required coverage for low-complexity UE:s and other UE:s operating delay tolerant machine-type communication (MTC) applications, time repetition techniques may be used, which enables energy accumulation of the received signals at the UE side.
Discussions are currently ongoing (e.g. within 3GPP and MFA), regarding the potential to evolve existing unlicensed standards to also support IoT use-cases within unlicensed bands. One issue to consider for such work is the regulatory requirements, which may differ, e.g. depending on frequency band and geographical region. One frequency band that may be eligible for IoT operation is the band in the vicinity of 2.4 GHz. Requirements for the European region are specified within the ETSI harmonized standard for equipment using wideband modulation (ETSI EN 300 328).
ETSI EN 300 328 provisions several adaption requirements for different operation modes. From the top level, equipment can be classified either as frequency hopping or non-frequency hopping, as well as adaptive or non-adaptive. Adaptive equipment is mandated to sense whether the channel is occupied in order to better coexist with other users of the channel. The receiving node will be unaware of the result of the sensing and therefore needs to detect whether any signal is present or not. While such signal detection would most likely be feasible for devices operating at moderate to high SINR levels, they may be infeasible for very low signal-to-interference and noise ratio (SINR) levels (which may be applicable in IoT scenarios). In particular, for systems using repetition schemes to achieve coverage extension, the received SINR of each individual transmission is typically very low. The effective SINR may increase through accumulation of multiple transmissions, but when the accumulation includes both signal and noise (as could be the case when the transmitter uses adaptive mechanisms) the repetition techniques may fail. Thus, an IoT standard for 2.4 GHz in Europe may benefit from avoiding the adaptive approach of equipment classification. Further, non-frequency hopping equipment is subject to requirements on maximum power spectral density (PSD) of 10 dBm/MHz, thus limiting the maximum output power for systems using narrow bandwidths. Thus, an IoT standard for 2.4 GHz in Europe may benefit from avoiding the non-frequency hopping approach of equipment classification.
Requirements for non-adaptive frequency hopping include the following:                A maximum Tx-sequence-time of 5 ms. Tx-sequence is defined as a period in time during which a single or multiple transmissions may occur and which shall be followed by a Tx-gap.        A minimum duration of the transmission gap of 5 ms.        A maximum accumulated transmit time of 15 ms, which is the maximum total transmission time a node may be allowed to use one frequency before moving to the next frequency hop.        A maximum Output Power of 20 dBm, i.e. 100 mW.        A maximum Medium Utilization (MU) of 10%, where the medium utilization is defined as, MU=(P/100 mW)*DC, where P is the transmission power (Pout) and DC is the Duty Cycle which in turn is defined as DC=(Transmitter ‘on’ time)/(Observation Period). Here the Observation Period is defined as the (Average Dwelling time*max (100; 2* number of hopping frequencies)). A dwelling time is the time spent on a frequency before moving to another frequency.        The maximum allowed dwelling time is maximum accumulated transmit time+transmission time gaps between, i.e. 30 ms.        
Further analyzing the requirements, it is understood that the Medium Utilization requirement limits the Duty Cycle with a dependency on output power (transmission power). When a device is transmitting at the maximum output power level of 20 dBm, the duty-cycle limitation limits the total Transmitter ‘on’ time to 10% of the duration of the Observation Period. If the output power is lowered below the maximum output power, the allowed duty-cycle increases in proportion to the decrease in power. The resulting requirement on duty-cycle for different exemplary output powers (given the medium utilization requirement of 10%) is illustrated via a few example values in the following table:
Pout (dBm)Duty Cycle2010%1720%1440%1180%
Therefore, in relation to this kind of scenario (and other relevant scenarios) there is a need for approaches that address the limitation dependency between transmission power level and duty cycle.